The present invention relates to a method of controlling an engine exhaust gas system which can efficiently remove nitrogen oxide in the exhaust gas of a lean burn engine by using a nitrogen oxide sensor and a nitrogen oxide adsorbing means when the nitrogen oxide contained in the exhaust gas of an internal combustion engine is treated, and to a method of detecting the deterioration of a catalyst/adsorbing means used in the former controlling method.
Conventionally, a method is disclosed, in which nitrogen oxide (hereinafter referred to as NOx) is adsorbed to a three-way catalyst disposed in an exhaust system in a lean burn engine, and the NOx is released and reduced by intermittently flowing stoichiometric or rich exhaust gas to the adsorbed nitrogen oxide, for example, in a U.S. Society of Automotive Engineers paper (SAE paper) 952490.
However, in this system, since whether the exhaust gas is made stoichiometric or rich, or the timing of fuel injection and the amount of injection are set values in accordance with driving conditions, inaccuracy can not be avoided such that for the actual adsorption amount of NOx, the timing of injection is too early or too late, or the amount of injection is too large or too small. Thus, the system has such disadvantages that if the timing of injection is too early, fuel efficiency is lowered, and if the timing is too late, a large amount of NOx is exhausted. Further, when an NOx catalyst is deteriorated, a large amount of NOx is exhausted, however, there has been no high reliable method of detecting the deterioration thereof.